1. Field of the Invention
This invention relates to a novel process for etching metal. More particularly, this invention relates to a novel copper etching process which is particularly useful in the manufacture of printed circuit boards.
2. Brief Description of the Prior Art
Printed circuit boards are widely used by the modern electronics industry as interconnecting devices. Prior art manufacturing processes are characterized as subtractive, additive or as semi-additive processes. Of these processes, the subtractive and the semi-additive processes require removal of unprotected copper from the insulating board substrate. An example of a subtractive process for a single sided board is one in which a copper foil, usually relatively thick, is applied to an insulating base substrate, desired mounting holes are formed therein, and an etch resist material, usually a tin/lead solder, is applied or formed over the copper layer in the positive image of the desired circuit configuration. The board is immersed in a copper etchant that removes the copper from the areas unprotected by the resist, leaving the desired circuit configuration. Because the copper layer is relatively thick, a strong etchant and considerable time is required to accomplish the fabrication. Where plated through holes are desired, the foregoing substractive process can be modified in an "additive" way by forming holes, and then electrolessly plating over the copper covered laminate to coat the thick copper layer and the hole walls. A plating resist is then applied over the electroless copper surface in the negative image of the desired patterns and copper electrolytically applied to the desired thickness. Then a dissimilar metal-resist is applied over the portions of the copper surface to be retained typically by electrolytic plating of a nickel metal or a solder metal (tin/lead) to cover the plated hole walls as well, and in the subsequent step of etching, generally only the undesired copper is removed.
In the semi-additive process, through-holes are formed in an insulating substrate, followed by activation of the substrate and the through-holes with a noble metal ion activating solution. This is followed by electroless deposition of copper on the top of the substrate and on the walls of the formed holes through use of plating solution. Thereafter, a first resist, such as a photoresist or screen printable resist, is applied to the formed board in the negative image of the desired circuit pattern leaving exposed those regions of the electroless copper corresponding to the desired circuit pattern. A copper layer is then electroplated on top of the exposed electroless copper portions, including the hole walls. The first resist is removed and the structure is immersed for a time in a copper etchant which removes the uncovered electroless copper. Alternatively, the electroplated surfaces of the substrate can be electroplated with a second etch resist such as a silver, tin, lead or gold to cover the surfaces of the electroplated copper prior to removal of the first resist followed by removal of the first resist and etching of the copper. The copper underlying the second etch resist forms the desired circuit on the circuit board.
A problem encountered with solution etching used in conventional subtractive processes and to a lesser extent in conventional semi-additive processes is undercutting at the edges of the conductive paths. Etch factors of 2.0 are normal for solution etching, i.e., the etching process proceeds two times as fast in the vertical directions as it does in the lateral direction. Undercutting makes it difficult for the circuit board manufacturer to control conductor width and profile, and is thus one of the major factors limiting the resolution attainable in printed circuit boards. Solution etching also promotes galvanic corrosion, due to difference in potential between the etch resist, usually solder, and the copper, which further compounds the resultant conductor profile. Control of conductor profile is a severe problem not only in production of high resolution printed circuit boards, but also in production of circuit boards for controlled impedence and microwave applications.
About a decade ago the integrated circuit industry faced a similar problem of undercutting during solution etching. This was overcome by the development of dry etching techniques such as plasma etching, ion milling, sputter etching, and reactive ion etching. These processes must be carried out in a vacuum chamber. Material is removed by formation of volatile products from reaction with selectively reactive gaseous species generated in-situ (plasma etching), by bombardment of the wafer surface with accelerated ions (ion milling and sputter etching), or by a combination of both (reactive ion etching). These techniques are unsuited for etching printed circuit boards for several reasons. First, very large and expensive vacuum systems would be required because of the large size of printed circuit boards. Second, the etch rates in these procedures are much too low for economical application of the procedures to the manufacture of printed circuit boards. Typical etch rates are two orders of magnitude lower than those common for printed circuit board etching. Third, copper which must be removed during etching of printed circuit boards does not form volatile products with the reactive species typically used for plasma etching. Therefore, copper is very difficult if not impossible to etch by this method.